1. Field of the Invention
The present invention relates to an electronic display device which is formed having an EL (electroluminescent) element on a substrate. More particularly, it relates to a display device which employs a semiconductor element (an element including a semiconductor thin film). Also, it relates to an electronic equipment whose display portion includes an EL display device.
2. Description of the Related Art
In recent years, EL display devices each having an EL element as a self-emission type element have been vigorously developed. The EL display device is also called the “organic EL display (OELD)” or “organic light emitting diode (OLED)”.
Unlike a liquid crystal display device, the EL display device is of self-emission type. The EL element has a construction wherein an EL layer is sandwiched in between a pair of electrodes (an anode and a cathode), and wherein the EL layer has a multilayer structure ordinarily. Typically mentioned is the multilayer structure of “hole transporting layer/light emitting layer/electron transporting layer” proposed by Tang et al., Eastman Kodak Company. The multilayer structure exhibits a very high emission efficiency, and most of the EL display devices being currently under researches and developments adopt this structure.
Alternatively, the multilayer structure may be so formed that the anode is successively overlaid with a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer, or a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer/an electron injecting layer. The light emitting layer may well be doped with a fluorescent coloring matter or the like.
In this specification, all layers interposed between the cathode and the anode shall be generally termed the “EL layer”. Accordingly, the hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer and electron injecting layer mentioned above are all included in the “EL layer”.
Herein, a predetermined voltage is applied to the EL layer of the above structure by the pair of electrodes, whereby light is emitted by the recombination of carriers taking place in the light emitting layer. In this specification, the light emission of the EL element shall be called the “drive of the EL element”. Also in this specification, a light emitting element which is constituted by the anode, EL layer and cathode shall be called the “EL element”.
Here, in this specification, the “EL element” shall cover both light emission (fluorescence) from a singlet exciton and light emission (phosphorescence) from a triplet exciton.
The driving systems of EL display devices include a passive system and an active system.
The passive type EL display device has a structure wherein stripe-like anodes (transparent electrodes) formed on an insulating substrate, organic EL layers, and stripe-like cathodes formed so as to intersect orthogonally to the anodes are stacked in succession. The equivalent circuit of the passive type EL display device is shown in FIG. 2. According to the passive system, one of scanning lines is selected, and among pixels lying on the selected scanning line, only ones whose signal lines are ON states emit light.
Signals to be inputted to the signal lines are created in such a way that a data signal (video signal) externally inputted is edited by a signal line driving circuit. Here in the passive type EL display device, the signal line driving circuit is disposed in such a way that an IC chip is mounted on the display device by TAB (Tape Automated Bonding), or that the IC chip is assembled in the display device by bonding it directly onto the pixel substrate. The IC chip is so constructed that a circuit is formed on a semiconductor substrate such as silicon chip. In the case of bonding the IC chip directly onto the pixel substrate, accordingly, the semiconductor substrate is bonded onto the insulating substrate used as the pixel substrate.
On the other hand, the active type EL display device is constructed of EL elements, and gate signal lines, source signal lines, power supply lines, transistors and capacitors which are formed on an insulating substrate. One of the capacitors and two of the transistors are disposed for each of the pixels of the display device.
In general, the active type EL display device has a structure wherein two or more transistors are disposed for each pixel.
In the active type EL display device, the pixel is formed in such a way that the transistors are fabricated on the insulating substrate by employing semiconductor thin films. Here, the transistors formed by employing the semiconductor thin films are called the “thin film transistors (hereinbelow, abbreviated to “TFTs”).
The circuit diagram of the pixel of the active type EL display device is shown in FIG. 3.
The gate electrode of the switching TFT is connected to the gate signal line. One of the source region and drain region of the switching TFT is connected with the source signal line, while the other is connected with the gate electrode of the EL driving TFT and one electrode of the capacitor. One of the source region and drain region of the EL driving TFT is connected with the anode or cathode of the EL element, while the other is connected with the power supply line. That electrode of the capacitor which is not connected with the switching TFT, is connected with the power supply line.
In the pixel for which both the gate signal line and the source signal line have turned ON, charges are stored in the capacitor through the switching TFT. During a time period for which the capacitor continues to apply a voltage to the gate electrode of the EL driving TFT, current continues to flow from the power supply line to the EL element through the EL driving TFT, and the EL element continues to emit light.
Signals to be inputted to the source signal lines are created in such a way that a data signal externally inputted is edited by a source signal line driving circuit. In the active type EL display device, the source signal line driving circuit can be fabricated of TFTs on the insulating substrate simultaneously with the circuits of the pixel portion of the display device.
An analog driving method (analog drive) is mentioned as the driving method of the active type EL display device. The analog drive will be explained with reference to FIGS. 4 and 5.
FIG. 4 shows the structure of the pixel portion of the active type EL display device conforming to the analog drive. Gate signal lines (G1-Gy) to which selection signals from a gate signal line driving circuit are respectively inputted, are connected to the gate electrodes of switching TFTs 1801 included in individual pixels. Besides, either regions of the source regions and drain regions of the switching TFTs 1801 included in the individual pixels are connected to source signal lines (also called “data signal lines”) (S1-Sx) to which analog signals are respectively inputted, while the other regions are connected to the gate electrodes of EL driving TFTs 1804 included in the individual pixels and also to capacitors 1808 included in the individual pixels.
Either regions of the source regions and drain regions of the EL driving TFTs 1804 included in the individual pixels are connected to power supply lines (V1-Vx), while the other regions are connected to EL elements 1806. The potentials of the power supply lines (V1-Vx) are called “power source potentials”. Besides, the power supply lines (V1-Vx) are connected to the capacitors 1808 included in the individual pixels.
Each of the EL elements 1806 includes an anode, a cathode, and an EL layer which is interposed between the anode and the cathode. In a case where the anode of the EL element 1806 is connected with the source region or drain region of the EL driving TFT 1804, this anode of the EL element 1806 serves as a pixel electrode, and the cathode thereof serves as a counter electrode. To the contrary, in a case where the cathode of the EL element 1806 is connected with the source region or drain region of the EL driving TFT 1804, the anode of the EL element 1806 serves as a counter electrode, and the cathode thereof serves as a pixel electrode.
Here in this specification, the potential of the counter electrode shall be called the “counter potential”. Also, a power source for applying the counter potential to the counter electrode shall be called the “counter power source”. The potential difference between the potential of the pixel electrode and that of the counter electrode is an “EL driving voltage”, which acts across the EL layer.
FIG. 5 shows a timing chart in the case where the active type EL display device depicted in FIG. 4 is driven by the analog driving method. A time period from the selection of one of the gate signal lines till the next selection of another, is called “one line period (L)”. Besides, a time period from the display of one image till the display of the next image corresponds to “one frame period (F)”. In the case of the active type EL-display device depicted in FIG. 4, the gate signal lines are laid in the number v, and hence, y line periods (L1-Ly) are provided within one frame period.
As the resolution of the display device becomes higher, the number of the line periods within one frame period enlarges more, and the driving circuit must be driven at a higher frequency.
First, the power supply lines (V1-Vx) are held at a predetermined power source potential. Also, the counter potential being the potential of each counter electrode is held at a predetermined potential. The counter potential has a potential difference from the power source potential to the extent that the EL element emits light.
In the first line period (L1), the selection signal from the gate signal line driving circuit is inputted to the gate signal line G1. Besides, the analog signals are successively inputted to the source signal lines (S1-Sx). Since all the switching TFTs connected to the gate signal line G1 fall into their ON states, the analog signals inputted to the source signal lines are respectively inputted to the gate electrodes of the EL driving TFTs through the corresponding switching TFTs.
Here, the “ON state” of each TFT shall signify that the source—drain path of the TFT has been brought into a conductive state by the gate voltage of the TFT.
The quantity of current flowing through the channel forming region of the EL driving TFT is controlled by the magnitude of the potential (the voltage) of the signal which is inputted to the gate electrode of this TFT. Therefore, the potential acting on the pixel electrode of the EL element is determined by the magnitude of the potential of the analog signal which is inputted to the gate electrode of the EL driving TFT. That is, the EL element emits the light under the control by the potential of the analog signal.
When the analog signals have been inputted to all the source signal lines (S1-Sx) by iterating the above operations, the first line period (L1) ends. Incidentally, one line period may well be set at a sum obtained by adding a horizontal retrace period to the time period in which the analog signals are inputted to all the source signal lines (S1-Sx). Subsequently, the second line period (L2) starts, and the selection signal is inputted to the gate signal line G2. Besides, the analog signals are successively inputted to the source signal lines (S1-Sx) as in the first line period (L1).
In due course, when the selection signals have been inputted to all the gate signal lines (G1-Gy), all the line periods (L1-Ly) end. When all the line periods (L1-Ly) have ended, one frame period ends. During one frame period, all the pixels present displays, and one image is formed. Incidentally, one frame period may well be set at a sum obtained by adding a vertical retrace period to all the line periods (L1-Ly).
In the above way, the quantity of light emission of the EL element is controlled by the analog signal, and a gradation display is presented by the control of the quantity of light emission. In this method, the gradation display is presented by variation in the potential of the analog signal which is inputted to the source signal line.
FIG. 6A is a graph showing the transistor characteristic of the EL driving TFT. The transistor characteristic 401 is called the “Id-Vg characteristic” (or “Id-Vg curve”). Here, symbol Id denotes a drain current, and symbol Vg denotes a gate voltage. The quantity of current which flows versus any gate voltage can be known from the graph.
Usually, the region of the Id-Vg characteristic as indicated by a broken line 402 is used in driving the EL element. The enlarged diagram of the enclosed region 402 is shown in FIG. 6B.
In FIG. 6B, a hatched region is called a “subthreshold region”. In actuality, the subthreshold region signifies a region which corresponds to the gate voltage equal to or lower than a threshold voltage (Vth). In this region, the drain current varies exponentially versus the variation of the gate voltage. A current control based on the gate voltage is performed using the region.
The analog signal inputted into the pixel by the turn-ON of the switching TFT becomes the gate voltage of the EL driving TFT. On this occasion, the drain current is determined in one-to-one correspondence with the gate voltage in accordance with the Id-Vg characteristic shown in FIG. 6A. More specifically, the potential of the drain region (an EL driving potential in the ON state) is determined in correspondence with the voltage of the analog signal inputted to the gate electrode of the EL driving TFT, the predetermined drain current flows through the EL element, and the EL element emits the light in the quantity of light emission corresponding to the quantity of the current.
As explained above, the quantity of light emission of the EL element is controlled by the analog signal, and the gradation display is presented.
With the passive type EL display device, in the case of employing the TAB for assembling the signal line driving circuit, there is the problem that an area required for the TAB is difficult to be made small, so reduction in the size of the display device is difficult. Meanwhile, when the IC chip is bonded directly onto the substrate formed with the pixel portion, the surface of the bonding between the semiconductor substrate of the IC chip and the insulating substrate of the pixel portion becomes an interface where different kinds of substances are bonded. This poses the problem that a distortion is induced at the interface by a temperature change on account of the difference between the thermal expansion coefficients of the substances. The distortion disturbs the structure of the driving circuit, and forms one of causes for spoiling the reliability of the passive type EL display device.
On the other hand, with the active type EL display device, the source signal line driving circuit can be fabricated on the insulating substrate simultaneously with the circuits of the pixel portion. Therefore, the active type EL display device is free from the problems of the passive type one in the case of assembling the source signal line driving circuit. Regarding the construction of the pixel portion, however, the two transistors are arranged every pixel. This poses the problem that, as the pixel becomes smaller, a proportion occupied by the transistors within the pixel enlarges more, so the aperture factor of the pixel, in turn, the display device lessens.